Electroencephalography

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Electroencephalography

Definition

Electroencephalography, or EEG, is a neurological test that involves attaching electrodes to the head of a person to measure and record electrical activity in the brain over time.

Purpose

The EEG, also known as a brain wave test, is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and diseases such as strokes, tumors, encephalitis, mental retardation, and sleep disorders. The results of the test can distinguish psychiatric conditions such as schizophrenia, paranoia, and depression from degenerative mental disorders such as Alzheimer's and Parkinson's diseases. An EEG may also be used to monitor brain activity during surgery to assess the effects of anesthesia. It is also used to determine brain status and brain death.

Demographics

The number of EEG tests performed each year can only be estimated. It is not a reportable event and is used in the diagnostic workup for a number of disorders. The number of EEG tests per year is estimated to be in the range of 10–25 million.

Description

Before an EEG begins, a nurse or technologist attaches approximately 16–21 electrodes to a person's scalp
using an electrically conductive, washable paste. The electrodes are placed on the head in a standard pattern based on head circumference measurements. Depending on the purpose for the EEG, implantable, or invasive, electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone. Depth electrodes, or subdural strip electrodes, are surgically implanted into the brain and are used to localize a seizure focus in preparation for epilepsy surgery. Once in place, even implantable electrodes do not cause pain. The electrodes are used to measure the electrical activity in various regions of the brain over the course of the test period.

For the test, a person lies on a bed, padded table, or comfortable chair and is asked to relax and remain still while measurements are being taken. An EEG usually takes no more than one hour, although long-term monitoring is often used for diagnosis of seizure disorders. During the test procedure, a person may be asked to breathe slowly or quickly. Visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalography unit makes a continuous graphic record of the person's brain activity, or brain waves, on a long strip of recording paper or computer screen. This graphic record is called an electroencephalogram. If the display is computerized, the test may be called a digital EEG, or dEEG.

The sleep EEG uses the same equipment and procedures as a regular EEG. Persons undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours, or up to eight or nine hours if it is a night's sleep.

In an ambulatory EEG, individuals are hooked up to a portable cassette recorder. They then go about normal activities and take normal rest and sleep for a period of up to 24 hours. During this period, individuals and their family members record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.

An extension of the EEG technique, called quantitative EEG (qEEG), involves manipulating the EEG signals with a computer using the fast Fourier transform algorithm. The result is then best displayed using a colored gray scale transposed onto a schematic map of the head to form a topographic image. The brain map produced in this technique is a vivid illustration of electrical activity in the brain. This technique also has the ability to compare the similarity of the signals between different electrodes, a measurement known as spectral coherence. Studies have shown the value of this measurement in diagnosis of Alzheimer's disease and mild closed head injuries. The technique can also identify areas of the brain having abnormally slow activity when the data are both mapped and compared to known normal values. The result is then known as a statistical or significance probability map (SPM). This allows differentiation between early dementia (increased slowing) or otherwise uncomplicated depression (no slowing).

Diagnosis/Preparation

An EEG is generally performed as one test in a series of neurological evaluations. Rarely does the EEG form the sole basis for a particular diagnosis.

Full instructions should be given to individuals receiving an EEG when they schedule their test. Typically, individuals taking medications that affect the central nervous system, such as anticonvulsants, stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one to two days). However, such requests should be cleared with the treating physician. EEG test candidates may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. They may also be asked to arrive for the test with clean hair that is free of styling products to make attachment of the electrodes easier.

Individuals undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep.

Aftercare

If an individual has suspended regular medication for the test, the EEG nurse or technician should advise as to when to begin taking it again.

Risks

Being off certain medications for one to two days may trigger seizures. Certain procedures used during EEG may trigger seizures in persons with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic for epilepsy (i.e., to determine the type of seizures an individual is experiencing) this may be a desired effect, although the person needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG.

Normal results

In reading and interpreting brain wave patterns, a neurologist or other physician will evaluate the type of brain waves and the symmetry, location, and consistency of brain wave patterns. Brain wave response to certain stimuli presented during the EEG test (such as flashing lights or noise) will also be evaluated.

The four basic types of brain waves are alpha, beta, theta, and delta, with the type distinguished by frequency. Alpha waves fall between 8 and 13 Hertz (Hz), beta are above 13 Hz, theta between 4 and 7 Hz, and delta are less than 4 Hz. Alpha waves are usually the dominant rhythm seen in the posterior region of the brain in older children and adults, when awake and relaxed. Beta waves are normal in sleep, particularly for infants and young children. Theta waves are normally found during drowsiness and sleep and are normal in wakefulness in children, while delta waves are the most prominent feature of the sleeping EEG. Spikes and sharp waves are generally abnormal; however, they are common in the EEG of normal newborns.

Different types of brain waves are seen as abnormal only in the context of the location of the waves, a person's age, and one's conscious state. In general, disease typically increases slow activity, such as theta or delta waves, but decreases fast activity, such as alpha and beta waves.

Not all decreases in wave activity are abnormal. The normal alpha waves seen in the posterior region of the brain are suppressed merely if a person is tense. Sometimes the addition of a wave is abnormal. For example, alpha rhythms seen in a newborn can signify seizure activity. Finally, the area where the rhythm is seen can be telling. The alpha coma is characterized by alpha rhythms produced diffusely, or, in other words, by all regions of the brain.

Some abnormal beta rhythms include frontal beta waves that are induced by sedative drugs. Marked asymmetry in beta rhythms suggests a structural lesion on the side lacking the beta waves. Beta waves are also commonly measured over skull lesions, such as fractures or burr holes, in activity known as a breach rhythm.

Usually seen only during sleep in adults, the presence of theta waves in the temporal region of awake, older adults has been tentatively correlated with vascular disease. Another rhythm normal in sleep, delta rhythms, may be recorded in the awake state over localized regions of cerebral damage. Intermittent delta rhythms are also an indication of damage of the relays between the deep gray matter and the cortex of the brain. In adults, this intermittent activity is found in the frontal region whereas in children, it is in the occipital region.

The EEG readings of persons with epilepsy or other seizure disorders display bursts, or spikes, of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, multifocal epilepsy may be present. The EEG can be used to localize the region of the brain where the abnormal electrical activity is occurring. This is most easily accomplished using a recording method, or montage, called an average reference montage. With this type of recording, the signal from each electrode is compared to the average signal from all the electrodes. The negative amplitude (upward movement, by convention) of the spike is observed for the different channels, or inputs, from the various electrodes. The negative deflection will be greatest as recorded by the electrode that is closest in location to the origin of the abnormal activity. The spike will be present but of reduced amplitude as the electrodes move farther away from the site producing the spike. Electrodes distant from the site will not record the spike occurrence.

A final variety of abnormal result is the presence of slower-than-normal wave activity, which can either be a slow background rhythm or slow waves superimposed on a normal background. A posterior dominant rhythm of 7 Hz or less in an adult is abnormal and consistent with encephalopathy (brain disease). In contrast, localized theta or delta rhythms found in conjunction with normal background rhythms suggest a structural lesion.

Morbidity and mortality rates

There are few adverse conditions associated with an EEG test. Persons with seizure disorders may induce seizures during the test in reaction to flashing lights or by deep breathing. Mortality from an EEG has not been reported.

Alternatives

There are no equivalent tests that provide the same information as an EEG.

WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?

Electroencephalography is often administered by specially trained technicians who are supervised by a neurologist or other physician with specialized training in administering and interpreting the test. Because of the equipment involved, an EEG is usually administered in a hospital setting. It may be conducted in a professional office.

QUESTIONS TO ASK THE DOCTOR

How many EEG procedures has the technician performed?

What preparations are being made to treat an induced seizure?

Is the supervising physician appropriately certified to interpret an EEG?

Cite this article Pick a style below, and copy the text for your bibliography.

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Electroencephalography

Electroencephalography

Definition

Electroencephalography, or EEG, is a neurological test that involves attaching electrodes to the head of a person to measure and record electrical activity in the brain over time.

Purpose

The EEG, also known as a brain wave test, is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and diseases such as strokes, tumors, encephalitis, mental retardation , and sleep disorders. The results of the test can distinguish psychiatric conditions such as schizophrenia , paranoia, and depression from degenerative mental disorders such as Alzheimer's and Parkinson's diseases. An EEG may also be used to monitor brain activity during surgery to assess the effects of anesthesia. Additionally, it is used to determine brain status and brain death.

Precautions

There are few adverse conditions associated with an EEG test. Persons with seizure disorders may experience seizures during the test in reaction to flashing lights or by deep breathing.

Description

Before an EEG begins, a nurse or technologist attaches approximately 16–21 electrodes to a person's scalp using an electrically conductive, washable paste. The electrodes are placed on the head in a standard pattern based on head circumference measurements. Depending on the purpose for the EEG, implantable, or invasive, electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone. Depth electrodes, or subdural strip electrodes, are surgically implanted into the brain and are used to localize a seizure focus in preparation for epilepsy surgery. Once in place, even implantable electrodes do not cause pain . The electrodes are used to measure the electrical activity in various regions of the brain over the course of the test period.

For the test, a person lies on a bed, padded table, or comfortable chair and is asked to relax and remain still while measurements are being taken. An EEG usually takes no more than one hour, although long-term monitoring is often used for diagnosis of seizure disorders. During the test procedure, a person may be asked to breathe slowly or quickly. Visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalography unit makes a continuous graphic record of the person's brain activity, or brain waves, on a long strip of recording paper or computer screen. This graphic record is called an electroencephalogram. If the display is computerized, the test may be called a digital EEG, or dEEG.

The sleep EEG uses the same equipment and procedures as a regular EEG. Persons undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours, or up to eight or nine hours if it is a night's sleep.

In an ambulatory EEG, individuals are hooked up to a portable cassette recorder. They then go about normal activities and take normal rest and sleep for a period of up to 24 hours. During this period, individuals and their family members record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.

An extension of the EEG technique, called quantitative EEG (qEEG), involves manipulating the EEG signals with a computer using the fast Fourier transform algorithm. The result is then best displayed using a colored gray scale transposed onto a schematic map of the head to form a topographic image. The brain map produced in this technique is a vivid illustration of electrical activity of the brain. This technique also has the ability to compare the similarity of the signals between different electrodes, a measurement known as spectral coherence. Studies have shown the value of this measurement in diagnosis of Alzheimer's disease and mild closed-head injuries. The technique can also identify areas of the brain having abnormally slow activity when the data are both mapped and compared to known normal values. The result is then known as a statistical or significance probability map (SPM). This allows differentiation between early dementia (increased slowing) or otherwise uncomplicated depression (no slowing).

Preparation

An EEG is generally performed as one test in a series of neurological evaluations. Rarely does the EEG form the sole basis for a particular diagnosis.

Full instructions should be given to individuals receiving an EEG when they schedule their test. Typically, individuals taking medications that affect the central nervous system , such as anticonvulsants , stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one or two days). However, such requests should be cleared with the treating physician. EEG test candidates may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. They may also be asked to arrive for the test with clean hair that is free of spray or other styling products to make attachment of the electrodes easier.

Individuals undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep.

Aftercare

If an individual has suspended regular medication for the test, the EEG nurse or technician should advise as to when to begin taking it again.

Risks

Being off certain medications for one to two days may trigger seizures. Certain procedures used during EEG may trigger seizures in persons with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic tool for epilepsy (i.e., to determine the type of seizures an individual is experiencing), this may be a desired effect, although the person needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG.

Normal results

In reading and interpreting brain wave patterns, a neurologist or other physician will evaluate the type of brain waves and the symmetry, location, and consistency of brain wave patterns. Brain wave response to certain stimuli presented during the EEG test (such as flashing lights or noise) will also be evaluated.

The four basic types of brain waves are alpha, beta, theta, and delta, with the type distinguished by frequency. Alpha waves fall between 8 and 13 Hertz (Hz), beta are above 13 Hz, theta between 4 and 7 Hz, and delta are less than 4 Hz. Alpha waves are usually the dominant rhythm seen in the posterior region of the brain in older children and adults, when they are awake and relaxed. Beta waves are normal in sleep, particularly for infants and young children. Theta waves are normally found during drowsiness and sleep and are normal in wakefulness in children, while delta waves are the most prominent feature of the sleeping EEG. Spikes and sharp waves are generally abnormal; however, they are common in the EEG of normal newborns.

Different types of brain waves are seen as abnormal only in the context of the location of the waves, a person's age, and one's state of consciousness. In general, disease typically increases slow activity such as theta or delta waves, but decreases fast activity such as alpha and beta waves.

Not all decreases in wave activity are abnormal. The normal alpha waves seen in the posterior region of the brain are suppressed merely if a person is tense. Sometimes the addition of a wave is abnormal. For example, alpha rhythms seen in a newborn can signify seizure activity. Finally, the area where the rhythm is seen can be telling. The alpha coma is characterized by alpha rhythms produced diffusely, or, in other words, by all regions of the brain.

Some abnormal beta rhythms include frontal beta waves that are induced by sedative drugs. Marked asymmetry in beta rhythms suggests a structural lesion on the side lacking the beta waves. Beta waves are also commonly measured over skull lesions such as fractures or burr holes, in an activity known as a breach rhythm.

Usually seen only during sleep in adults, the presence of theta waves in the temporal region of awake, older adults has been tentatively correlated with vascular disease. Another rhythm normal in sleep, delta rhythms, may be recorded in a wakeful state over localized regions of cerebral damage. Intermittent delta rhythms are also an indication of damage of the relays between the deep gray matter and the cortex of the brain. In adults, this intermittent activity is found in the frontal region, whereas in children it is in the occipital region.

The EEG readings of persons with epilepsy or other seizure disorders display bursts, or spikes, of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, multifocal epilepsy may be present. The EEG can be used to localize the region of the brain where the abnormal electrical activity is occurring. This is most easily accomplished using a recording method, or montage, called an average reference montage. With this type of recording, the signal from each electrode is compared to the average signal from all the electrodes. The negative amplitude (an upward movement) of the spike is observed for the different channels, or inputs, from the various electrodes. The negative deflection will be greatest as recorded by the electrode that is closest in location to the origin of the abnormal activity. The spike will be present but of reduced amplitude as the electrodes move farther away from the site producing the spike. Electrodes distant from the site will not record the spike occurrence.

A final variety of abnormal result is the presence of slower-than-normal wave activity, which can either be a slow background rhythm or slow waves superimposed on a normal background. A posterior dominant rhythm of 7 Hz or less in an adult is abnormal and consistent with encephalopathy (brain disease). In contrast, localized theta or delta rhythms found in conjunction with normal background rhythms suggest a structural lesion.

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Electroencephalogram

Electroencephalogram

Definition

An electroencephalogram (EEG), also called a brain wave test, is a diagnostic test which measures the electrical activity of the brain (brain waves) using highly sensitive recording equipment attached to the scalp by fine electrodes.

Purpose

EEG is performed to detect abnormalities in the electrical activity of the brain which may help diagnose the presence and type of various brain disorders, to look for causes of confusion, and to evaluate head injuries, tumors, infections, degenerative diseases, and other disturbances that affect the brain. The test is also used to investigate periods of unconsciousness. EEG may also confirm brain death in someone who is in a coma. EEG cannot be used to measure intelligence or diagnose mental illness. Specifically, EEG is used to diagnose the following:

seizure disorders (such as epilepsy or convulsions)

structural brain abnormality (such as a brain tumor or brain abscess)

head injury, encephalitis (inflammation of the brain)

hemorrhage (abnormal bleeding caused by a ruptured blood vessel)

cerebral infarct (tissue that is dead because of a blockage of the blood supply)

sleep disorders (such as narcolepsy)

Description

Brain cells communicate by producing tiny electrical impulses, also called brain waves. These electrical signals have certain rhythms and shapes, and EEG is a technique that measures, records, and analyzes these signals to help make a diagnosis. Electrodes are used to detect the electrical signals. They come in the shape of small discs that are applied to the head and connected to a recording device. The recording machine then converts the electrical signals into a series of wavy lines that are drawn onto a moving piece of graph paper. An EEG test causes no discomfort. Although having electrodes pasted on the skin may feel strange, they only record activity and do not produce any sensation. The patient needs to lie still with eyes closed because any movement can affect results. The patient may also be asked to do certain things during the EEG recording, such as breathing deeply and rapidly for several minutes or looking at a bright flickering light.

An EEG is performed by an EEG technician in a specially designed room that may be in the doctor's office or at a hospital. The patient is asked to lie on a bed or in a comfortable chair so that a relaxed EEG recording can be done. The technician either measures the scalp and marks the spots where small discs (electrodes) will be placed or fits the head with a special cap containing between 16 and 25 of these discs. The scalp is then rubbed with a mild, scratchy cleanser that may cause mild discomfort for a short while. The discs are attached to the body with a cream or gel. Alternatively, the technician may secure the discs to the skin with an adhesive. The heart may also be monitored during the procedure.

Precautions

Before an EEG, care should be taken to avoid washing hair with an oily scalp product 24 hours before the test. Doctors usually recommend that patients eat a meal or light snack some four hours before the test. Caffeinated drinks should be avoided for eight hours before the test. Sometimes, the EEG gives better results when the patient has had less than the usual amount of sleep. The doctor may ask that the child be kept awake for all or part of the night before the EEG. The healthcare provider may also discontinue some medications before the test.

Preparation

The physical and psychological preparation required for this test depends on the child's age, interests, previous experiences, and level of trust. For older children, research has shown that preparing ahead can reduce crying or resisting the test. In addition, children report less pain and show less distress when prepared. Proper preparation for the test can reduce a child's anxiety , encourage cooperation, and help develop coping skills.

Some general guidelines for preparing a toddler or preschooler for an EEG include the following:

Explain the EEG procedure in words that the child understands, avoiding abstract terminology.

Ensure that the child understands the exact body part involved and that the procedure will be limited to that area.

Describe how the test is likely to feel.

Give the child permission to yell, cry, or otherwise express any pain or discomfort verbally.

Stress the benefits of the EEG procedure and list things that the child may find pleasurable after the test, such as feeling better or going home.

The above guidelines also apply to school age children. Additionally, for older children, parents can try the following:

Include the child in the decision-making process, such as the time of day where the EEG is performed.

Suggest that the child hold the hand of the technician or someone else helping with the procedure.

As for adolescents, detailed information about the EEG should be provided and the reasons for the procedure should be explained in correct medical terminology. When the EEG is required for a seizure disorder , there is the potential risk that the test will trigger a seizure. This possibility should be openly discussed. Adolescents commonly have high concerns about risks and the best way to prepare them is to fully inform them. The healthcare provider could also be asked to limit the number of
strangers entering and leaving the room during the EEG procedure, since they can raise the patient's anxiety level.

Aftercare

There are no side effects or special procedures required after an EEG. The technician simply removes the gel with water and the adhesive, if used, with a special cleanser. Shampooing will rid the hair of any other material. A few patients are mildly sensitive to the gel or may get irritation from the rubbing of their scalps.

KEY TERMS

Electrode—A medium for conducting an electrical current.

Encephalitis—Inflammation of the brain, usually caused by a virus. The inflammation may interfere with normal brain function and may cause seizures, sleepiness, confusion, personality changes, weakness in one or more parts of the body, and even coma.

Epilepsy—A neurological disorder characterized by recurrent seizures with or without a loss of consciousness.

Hemorrhage—Severe, massive bleeding that is difficult to control. The bleeding may be internal or external.

Hyperventilation—Rapid, deep breathing, possibly exceeding 40 breaths/minute. The most common cause is anxiety, although fever, aspirin overdose, serious infections, stroke, or other diseases of the brain or nervous system. Also refers to a respiratory therapy involving deeper and/or faster breathing to keep the carbon dioxide pressure in the blood below normal.

Narcolepsy—A life-long sleep disorder marked by four symptoms: sudden brief sleep attacks, cataplexy (a sudden loss of muscle tone usually lasting up to 30 minutes), temporary paralysis, and hallucinations. The hallucinations are associated with falling asleep or the transition from sleeping to waking.

Seizure—A sudden attack, spasm, or convulsion.

Sleep disorder—Any condition that interferes with sleep. Sleep disorders are characterized by disturbance in the amount of sleep, in the quality or timing of sleep, or in the behaviors or physiological conditions associated with sleep.

Risks

The EEG test is very safe. However, if a patient has a seizure disorder, a seizure may be triggered by the flashing lights or hyperventilation. The healthcare
provider performing the EEG is trained to take care of the patient if this happens.

Normal results

An EEG returns normal results when brain waves have normal frequency and amplitude and other characteristics are typical.

Parental concerns

Before the test, parents should know that the child probably will cry, and restraints may be used. The most important way to help a child through an EEG procedure is by being there and caring. Crying is a normal response to the strange environment, unfamiliar people, restraints, and separation from the parent. Infants and young children will cry more for these reasons than because the test or procedure is uncomfortable. Knowing this from the onset may help parents feel less anxiety about what to expect. Having specific information about the test may further reduce anxiety.

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Electroencephalography

Electroencephalography

Definition

Electroencephalography (EEG) is a neurological diagnostic procedure that records the changes in electrical potentials (brain waves) in various parts of the brain.

Purpose

The EEG is an important aid in the diagnosis and management of epilepsy and other seizure disorders, as well as in the diagnosis of brain damage related to trauma and diseases such as strokes, tumors, encephalitis, and drug and alcohol intoxication. The EEG is also useful in monitoring brain wave activity and in the determination of brain death. Research is active in determining the role of EEG in the diagnosis and management of mental retardation , sleep disorders , degenerative diseases such as Alzheimer's disease and Parkinson's disease, and in certain mental disorders such as autism and schizophrenia .

Precautions

The EEG should be administered, monitored, and interpreted only by a specially trained health professional. It is important to recognize that diagnosis should not be based on the EEG alone—the EEG represents an adjunct to the neurological history, examination, and other specialized studies. The EEG is an extremely sensitive instrument, and tracings can be greatly influenced by the actions and the physiologic status of the patient. It is important that the patient be properly prepared physically and psychologically in order to obtain an accurate and reliable record. Medications such as anticonvulsants, tranquilizers, stimulants—including coffee, tea, cola drinks—and alcohol should be withheld for at least 24–48 hours prior to the test. Inasmuch as hypoglycemia affects brain wave patterns, the patient is told not to withhold any meals.

Description

Brain function is associated with electrical activity, which is always accompanied by an electrical field. This field consists of two parts, the electrical field and the magnetic field, and is called an electromagnetic field. The electrical field is measured by surface electrodes and is recorded by the electroencephalogram. Prior to the recording session, approximately 16–20 electrodes are attached to the patient's scalp with a conductive washable paste, or collodion. Depending on the purpose of the EEG, implantable needle electrodes may be utilized, in which case the patient should be informed that there will be mild discomfort.

The patient lies on a bed, padded table, or comfortable reclining chair and is asked to remain quiet and relaxed during the approximately one hour that is usually required. A sleep recording up to three hours in duration is usually obtained if the diagnosis is a seizure disorder. Under certain conditions, various stimuli such as flashing lights or deep breathing may be utilized. In an ambulatory EEG recording, the patient is attached to a portable cassette recorder and goes about regular activities, usually for up to 24 hours.

Magnetoencephalography

Magnetoencephalography, a supplement to EEG, also uses an electroencephalogram to measure the patient's electrical field. In addition, however, the patient's magnetic field is also recorded to measure electrical activity. Every electrical current generates a magnetic field. The magnetic field is detected by an instrument called a biomagnetometer and recorded as a magnetoencephalograph (MEG). The information provided by the MEG is entirely different from that provided by computed tomography (CT), topographic encephalography, or magnetic resonance imaging (MRI)—imaging instruments that provide still, structural, and anatomical information. The information recorded by the MEG provides important supplemental information to that recorded by the encephalogram and, used together and conjointly, they both provide a much more complete and comprehensive idea of cerebral events. Using MEG, the brain can be observed "in action" rather than just being viewed as a still image.

Magnetoencephalography has been used to map the sensory and motor cortices of the brain, to determine the organization of the auditory center of the brain, and to study cognitive functions such as speech, memory, attention and consciousness. This information is critical for neurosurgical planning such as the removal of brain lesions. Thus, preoperative MEG is valuable in planning the surgical treatment of tumors and malformations. MEG can provide surgeons with real-time computer-generated images of deep-seated lesions that are essential before surgery. The quantitative EEG is also known by the acronym BEAM (brain electrical activity mapping).

Preparation

Prior to the EEG, the patient is given full instructions in the procedure, particularly about the avoidance of certain medications and food. In cases where a sleep EEG is anticipated, the patient may be requested to minimize sleep or stay awake the night before the procedure. Sedatives to induce sleep should be avoided, if possible.

Aftercare

No specific procedures or aftercare are required. Patients are advised to resume their usual activities, especially the resumption of medications that had been temporarily discontinued.

Risks

The primary risk of EEG is the production of a seizure in an epileptic patient. This may result from the temporary discontinuation of anticonvulsant medication or from the provocation of a seizure by an epileptogenic stimulus such as flashing lights or deep breathing. Although the provocation of a seizure may serve to substantiate the diagnosis, all potential seizure patients should be carefully monitored to avoid injury in case a seizure does result.

Normal results

The rate, height, and length of brain waves vary depending on the part of the brain being studied, and every individual has an unique and characteristic brain-wave pattern. Age and state of consciousness also cause changes in wave patterns. Several wave patterns have been identified:

Alpha waves: Most of the recorded waves in a normal adult's EEG are the occipital alpha waves, which are best obtained from the back of the head when the subject is resting quietly with the eyes closed but not asleep. These waves, occurring typically in a pattern of eight to 13 cycles per second, are blocked by excitement or by opening the eyes.

Beta waves: These waves, obtained from the central and frontal parts of the brain, are closely related to the sensory-motor parts of the brain and are also blocked by opening the eyes. Their frequency is in the range of 8–30 hertz (cycles per second).

Delta waves: These are irregular, slow waves of 2–3 hertz and are normally found in deep sleep and in infants and young children. They indicate an abnormality in an awake adult.

Theta waves: These are characterized by rhythmic, slow waves of 4–7 hertz.

Abnormal results

EEG readings of patients with epilepsy or other seizure disorders display bursts, or spikes, of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres, multifocal epilepsy may be indicated.

Diagnostic brain-wave patterns of other disorders varies widely. The appearance of excess theta waves (four to eight cycles per second) may indicate brain injury. Brain wave patterns in patients with brain disease, mental retardation, and brain injury show overall slowing. A trained medical specialist should interpret EEG results in the context of the patient's medical history and other pertinent medical test results.

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Electroencephalography

Electroencephalography

Definition

Electroencephalography, or EEG, is a neurological test that uses an electronic monitoring device to measure and record electrical activity in the brain.

Purpose

The EEG is a key tool in the diagnosis and management of epilepsy and other seizure disorders. It is also used to assist in the diagnosis of brain damage and disease (e.g., stroke, tumors, encephalitis), mental retardation, sleep disorders, degenerative diseases such as Alzheimer's disease and Parkinson's disease, and certain mental disorders (e.g., alcoholism, schizophrenia, autism ).

An EEG may also be used to monitor brain activity during surgery and to determine brain death.

Precautions

Electroencephalography should be administered and interpreted by a trained medical professional only. Data from an EEG is only one element of a complete medical and/or psychological patient assessment, and should never be used alone as the sole basis for a diagnosis.

Description

Before the EEG begins, a nurse or technician attaches approximately 16-20 electrodes to the patient's scalp with a conductive, washable paste. Depending on the purpose for the EEG, implantable or invasive electrodes are occasionally used. Implantable electrodes include sphenoidal electrodes, which are fine wires inserted under the zygomatic arch, or cheekbone; and depth electrodes, which are surgically-implanted into the brain. The EEG electrodes are painless, and are used to measure the electrical activity in various regions of the brain.

For the test, the patient lies on a bed, padded table, or comfortable chair and is asked to relax and remain still during the EEG testing period. An EEG usually takes no more than one hour. During the test procedure, the patient may be asked to breathe slowly or quickly; visual stimuli such as flashing lights or a patterned board may be used to stimulate certain types of brain activity. Throughout the procedure, the electroencephalograph machine makes a continuous graphic record of the patient's brain activity, or brainwaves, on a long strip of recording paper or on a computer screen. This graphic record is called an electroencephalogram.

The sleep EEG uses the same equipment and procedures as a regular EEG. Patients undergoing a sleep EEG are encouraged to fall asleep completely rather than just relax. They are typically provided a bed and a quiet room conducive to sleep. A sleep EEG lasts up to three hours.

In an ambulatory EEG, patients are hooked up to a portable cassette recorder. They then go about their normal activities, and take their normal rest and sleep for a period of up to 24 hours. During this period, the patient and patient's family record any symptoms or abnormal behaviors, which can later be correlated with the EEG to see if they represent seizures.

Many insurance plans provide reimbursement for EEG testing. Costs for an EEG range from $100 to more than $500, depending on the purpose and type of test (i.e., asleep or awake, and invasive or non-invasive electrodes). Because coverage may be dependent on the disorder or illness the EEG is evaluating, patients should check with their individual insurance plan.

Preparation

Full instructions should be given to EEG patients when they schedule their test. Typically, individuals on medications that affect the central nervous system, such as anticonvulsants, stimulants, or antidepressants, are told to discontinue their prescription for a short time prior to the test (usually one to two days). Patients may be asked to avoid food and beverages that contain caffeine, a central nervous system stimulant. However, any such request should be cleared by the treating physician. Patients may also be asked to arrive for the test with clean hair free of spray or other styling products.

Patients undergoing a sleep EEG may be asked to remain awake the night before their test. They may be given a sedative prior to the test to induce sleep.

Aftercare

If the patient has suspended regular medication for the test, the EEG nurse or technician should advise him when he can begin taking it again.

Risks

Being off medication for one-two days may trigger seizures. Certain procedures used during EEG may trigger seizures in patients with epilepsy. Those procedures include flashing lights and deep breathing. If the EEG is being used as a diagnostic for epilepsy (i.e., to determine the type of seizures an individual is suffering from), this may be a desired effect, although the patient needs to be monitored closely so that the seizure can be aborted if necessary. This type of test is known as an ictal EEG.

Normal results

In reading and interpreting brainwave patterns, a neurologist or other physician will evaluate the type of brainwaves and the symmetry, location, and consistency of brainwave patterns. He will also look at the brainwave response to certain stimuli presented during the EEG test (such as flashing lights or noise). There are four basic types of brainwaves: alpha, beta, theta, and delta. "Normal" brainwave patterns vary widely, depending on factors of age and activity. For example, awake and relaxed individuals typically register an alpha wave pattern of eight to 13 cycles per second. Young children and sleeping adults may have a delta wave pattern of under four cycles per second.

Abnormal results

The EEG readings of patients with epilepsy or other seizure disorders display bursts or spikes of electrical activity. In focal epilepsy, spikes are restricted to one hemisphere of the brain. If spikes are generalized to both hemispheres of the brain, multifocal epilepsy may be present.

The diagnostic brainwave patterns of other disorders varies widely. The appearance of excess theta waves (four to eight cycles per second) may indicate brain injury. Brain wave patterns in patients with brain disease, mental retardation, and brain injury show overall slowing. A trained medical specialist should interpret EEG results in the context of the patient's medical history, and other pertinent medical test results.

Resources

BOOKS

Restak, Richard M. Brainscapes: An Introduction to What Neuroscience Has Learned About the Structure, Function, and Abilities of the Brain. NewYork: Hyperion, 1995.

KEY TERMS

Epilepsy— A neurological disorder characterized by recurrent seizures with or without a loss of consciousness.

Ictal EEG— Used to measure brain activity during a seizure. May be useful in learning more about patients who aren't responding to conventional treatments.

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electroencephalogram

electroencephalogram (EEG) Recording of electrical activity from the brains of animals was first reported by the British physiologist Caton in 1875. Berger, a German psychiatrist, described the human EEG in 1929, but it was only after a further description of ‘the Berger rhythm’, by Adrian and Matthews in Cambridge five years later, that it began to be used in research and diagnosis.

Electroencephalography records, from electrodes placed on the scalp, the weak electrical activity generated by the brain, its voltage ten times smaller than that from the heart displayed by an electrocardiogram (ECG). This makes the EEG very sensitive to interference from muscle activity in the scalp or from electronic equipment in the vicinity. Moreover, whereas in the heart the origin of the activity is very well understood, the EEG records the collective activity of large populations of neurons in the cerebral cortex. For all the sophistication of modern recording equipment and computerized analysis of the records, it has to be accepted that the EEG remains a relatively crude measuring device, although it enables recognition of the different phases of normal sleep, and is of diagnostic value in some abnormal conditions.

A number of electrodes (typically about 20) are positioned on the scalp and connected in pairs, yielding 8–16 channels, each recording the potential between two electrodes. Each electrode receives signals from an area of cortex of 2–3 cm diameter. But a third of the cortex is inaccessible, in the depths of the indentations (the sulci), on the basal surface, or hidden within the larger folds of the brain. Some 10–15 min of recording results in 20–30 pages of paper that can be bound and read like a book — and often also analyzed by computer.

Information lies in the frequency and amplitude (voltage) of the waves recorded in different channels. At rest, relaxed and with the eyes closed, the frequency of these waves is 8–12 Hz (cycles/sec). This ‘alpha’ activity is believed to reflect the brain in ‘idling’ mode, because if the person then either opens the eyes, or does mental arithmetic with the eyes closed, these waves disappear, to be replaced by irregular patterns (so-called desynchronized activity). In normal sleep there is characteristic higher voltage activity, in patterns which vary according to the level of sleep.

When there is severe diffuse brain abnormality, such as encephalitis or conditions causing coma, there will be usually be no alpha activity, whilst in the vegetative state there may be alpha activity that fails to desynchronize on eye opening. Faster frequencies (beta, at >13 Hz) or slower (theta, at 4–8 Hz) can be normal in infancy and childhood. Even slower ‘delta’ waves (<4 Hz) can be normal in sleep and in infancy, but in awake adults indicate severe abnormality. When localized they may indicate pathology such as a tumour or abscess in the brain, causing the adjacent cortex to produce abnormal rhythms; however, modern imaging techniques have replaced EEG as a means of detecting and locating such lesions.

It is in the investigation of epilepsy that EEG has proved most useful — both in diagnosing epilepsy as the cause of abnormal behaviours and in localizing the site in the brain from which abnormalities are originating. During an epileptic seizure there are bursts of high voltage activity in the region of the brain affected. The problem is the low probability of a patient having an attack during routine recording. However, the record in a person with epilepsy is often abnormal between attacks, and these abnormalities are more likely to be found if the patient hyperventilates (producing temporary alkalosis in the brain), or is fasting (with temporary mild hypoglycaemia). But some patients who never have a clinical seizure may show such abnormalities on EEG, whilst a third of patients who do have seizures have a normal record at times between attacks. To increase the chance of securing a recording during an attack patients may be fitted with electrodes that transmit by radio to a receiver. Such telemetry allows 24-hour recording during normal activities — which may also be monitored on video in order to visualize any seizure that occurs.

When patients with epilepsy are being investigated with a view to possible surgery the location of the seizure-producing lesion may be identified by using electrodes placed to cover areas not available to those placed on the scalp. Electrodes introduced through the cheek to the base of the skull can detect activity from the under surface of the brain, whilst those inserted through a burr hole (a small opening in the skull) on to the surface of deeper parts of the brain, or even into the substance of the brain, bring other areas under surveillance.

A visual, auditory, or somatic stimulus will normally evoke an altered wave form from the appropriate area of the cortex (for example the occipital region for a visual input). It is therefore possible with the EEG to explore the integrity of sensory pathways by means of these evoked potentials. This method is also used during operative procedures to help surgeons to identify such sensory pathways by observing the effects of electrical stimulation on the EEG, so as to avoid damaging them.

An occasional use of the EEG is in confirming the diagnosis of brain death, when an isoelectric (flat) record may be obtained, but technical difficulties can lead to equivocal results. In some places, but not in the UK, EEG is mandatory after clinical tests have been completed.

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Electroencephalogram (EEG)

Medical Discoveries
COPYRIGHT 1997 Thomson Gale

Electroencephalogram (EEG)

An electroencephalogram (EEG) is a graphic (vivid) picture of the electrical activity of the brain. An EEG is made by placing electrodes (small terminals which conduct an electrical current) on the subject's scalp and
connecting the electrode wires to a machine known as an electroencephalograph. The electroencephalograph then records the patterns of brain waves (rhythmic changes in the electric impulses of the brain.)

EEGs are used to diagnose epilepsy (a disorder marked by severe seizures or convulsions), brain tumors, strokes, and other neurological (nervous system) conditions. These conditions are characterized by distinctive, abnormal patterns of brain waves. EEGs are also used in investigating psychiatric disorders, such as schizophrenia (a psychiatric condition in which a person's sense of reality is severely distorted). In addition, EEGs help in defining brain death (the measurable end of brain activity). This diagnosis is necessary before the donation of organs for surgical transplants.

Making an Encephalogram

Hans Berger (1873-1941) was a German psychiatrist who developed the first human EEG in 1924. Berger was interested in psychophysiology (the study of the relationship between mental processes and the brain). Berger decided to measure the brain's electrical activity in the hope that the physiological record would provide insight into mental processes.

Berger began his search for the human EEG by experimenting with dogs. He moved on to humans and started placing needle electrodes under the scalp of patients who had lost some of their skull bones in surgery. It was while working with one of these patients that Berger recorded the first human EEG in 1924. At first, he was uncertain whether the oscillations (changes or variations) he had recorded originated in the brain. It was not until after he had conducted many other experiments that he published his first paper on the human electroencephalogram in 1929.

The initial reaction of others to Berger's work was one of disbelief. The scientific world doubted whether the workings of an organ as complex as the brain could be recorded through the skull. Berger did not achieve an international reputation until 1934. It was then that Edgar Douglas Adrian (1889-1977), a renowned English neurophysiologist (one who studies the functions of the nervous system), confirmed Berger's findings.

BEAM Enhances the Value of the EEG

Since the time of the original study, research scientists have used the EEG to identify the sources of brain activities. They have located the parts of the brain involved in the mental processes of reasoning, memory, and feeling. Interpreting the EEG was made easier in the 1980s with the use of the BEAM (brain electrical activity mapping) system. This system was invented by Frank Duffy of the Harvard Medical School. It uses computer
technology to combine the signals from the individual electrodes into a overall, color-coded map of the brain's electrical activity.

Using the computer, BEAM can handle a wide range of tasks. It can store large amounts of EEG data, compare healthy profiles with abnormal ones, and provide detailed analyses. These analyses have been used to accurately diagnose such conditions as dyslexia (a condition marked by a difficulty with reading) and schizophrenia. Both of these conditions are usually difficult to detect.

The Future of BEAM

Efforts are currently underway to use BEAM in matching EEG patterns to specific brain functions. For example, research scientists have used BEAM to map the electrical activity involved in the movement of a monkey's arm. The studies have shown that when the monkey anticipates moving its arm, the pattern of electrical activity in its brain changes. If efforts like these are successful, it may one day be possible to use computers and the electrical activity of the brain to control the movement of artificial limbs.

[See alsoArtificial limb and joint ; Transplant, surgical ]

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electroencephalography

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

electroencephalography (əlĕk´trōĕnsĕf´əlŏg´rafē), science of recording and analyzing the electrical activity of the brain. Electrodes, placed on or just under the scalp, are linked to an electroencephalograph, which is an amplifier connected to a mechanism that converts electrical impulses into the vertical movement of a pen over a sheet of paper. The recording traced by the pen is called an electroencephalogram (EEG). Readings may be obtained for a particular brain site by coupling a single electrode with an indifferent, or neutral, lead (monopolar technique) or between two areas of the brain through two independent electrodes (bipolar technique). The combination of impulses that are being recorded at any one time is called a montage.

Brainwave Patterns

The electrical activity of the brain was first demonstrated in 1929 by the German psychiatrist Hans Berger. The scientific professions were slow in giving proper attention to Berger's discovery of the brain rhythms he named alpha waves, but since then at least three other standard brainwave patterns have been isolated and identified. Alpha waves are fast, medium-amplitude oscillations, now known to represent the background activity of the brain in the physically and psychologically healthy adult. They are most characteristically visible during dream-sleep or when a subject is relaxing with eyes closed. Delta waves are large, slow-moving, regular waves, typically associated with the deepest levels of sleep. In children up to the age of puberty the appearance of high-amplitude theta waves, having a velocity between those of alpha and delta rhythms, usually signals the onset of emotional stimulation. The presence of theta waves in adults may be a sign of brain damage or of an immature personality. Beta rhythms are small, very fast wave patterns that indicate intense physiological stress, such as that resulting from barbiturate intoxification.

Uses of EEGs

By observing abnormalities in recordings and determining the area of the brain from which they originate, the physician's ability to diagnose and treat such conditions as epilepsy, cerebral tumor, encephalitis, and stroke, is greatly enhanced. Electroencephalograms have also proven valuable in the general study of brain physiology and in the particular study of sleep. Various types of Eastern meditation, e.g., yoga, use techniques that increase alpha and theta wave activity. Because of concomitant physiological changes during meditation, e.g., lessened anxiety, the techniques have recently become popular in the West. Using EEGs to enhance biofeedback, a subject can be taught to monitor and regulate his or her own brain waves; the technique has been used experimentally in control of epilepsy. EEGs are also used to determine brain death (see death).

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Electroencephalograph (EEG)

Gale Encyclopedia of Psychology
COPYRIGHT 2001 The Gale Group Inc.

Electroencephalograph (EEG)

A device used to record the electrical activity of the brain.

Electroencephalography is used for a variety of research and diagnostic purposes. It is usually conducted using electrodes, metal discs attached to the scalp or to wires connected to the skull or even to the brain itself. The signals obtained through the electrodes must then be amplified in order to be interpreted. EEG patterns typically take the form of waves, which may be measured according to both their frequency and size (also referred to as amplitude). The electrical activity of animals' brains had been recorded as early as 1875, but it was not until 1929 that the first human EEG was reported by Austrian psychiatrist Anton Berger. Since then, it has been used to study the effects of drugs on the brain, as well as the localization of certain behavioral functions in specific areas of the brain. EEGs have also been widely used in sleep research. While the deeper stages of sleep are characterized by large, slow, irregular brain waves, and, in some cases, bursts of high-amplitude waves called "sleep spindles," REM (rapid eye movement ) sleep, during which most vivid dreaming occurs, resembles the faster brain-wave pattern of the waking state.

As a diagnostic tool, EEGs have been used to diagnose epilepsy , strokes, infections, hemorrhages, inadequate blood supply to the brain, and certain tumors. They are especially useful because they can pinpoint the location of tumors and injuries to the brain. EEGs are also used to monitor patients in a coma and, during surgery, to indicate the effectiveness of anesthetics.

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electroencephalography

electroencephalography (i-lek-troh-en-sef-ă-log-răfi) n. the technique for recording the electrical activity from different parts of the brain and converting it into a tracing called an electroencephalogram (EEG). The machine that records this activity is known as an encephalograph. Electroencephalography is mostly used in the diagnosis and management of epilepsy and sleep disorders.

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electroencephalogram

electroencephalogram (EEG) Recording of electrical activity of the brain. Electrodes are attached to the scalp to pick up the tiny oscillating currents produced by brain activity. The visual trace is recorded on paper or an oscilloscope screen. The result is used mainly in the diagnosis and monitoring of epilepsy. The machine is called an electroencephalograph.

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electroencephalogram

electroencephalogram (EEG) A tracing or graph of the electrical activity of the brain. Electrodes taped to the scalp record electrical waves from different parts of the brain. The pattern of an EEG reflects an individual's level of consciousness and can be used to detect such disorders as epilepsy, tumours, or brain damage. See also brain death.

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In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.